[Solved] Populate select dropdown with correct number of days when select month.

why not use a date picker? (obviously depends on what you are trying to do)

https://jqueryui.com/datepicker/


If you still need a select with the number of days in the month use

http://php.net/manual/en/function.cal-days-in-month.php

you would need to invoke some sort of a function via ajax each time either the year or month select is changed

Date picker Not what I am after.

There's only one rule - please don't tell anyone to go and read the manual.  Sometimes the manual just SUCKS!